The present invention relates to a tin-zinc alloy electroplating bath and a method for electroplating using the same. In particular, the present invention relates to a tin-zinc alloy electroplating bath capable of stably forming a coating of a uniform alloy composition by keeping the plating alloy composition from the influence of the current density.
The tin-zinc alloy electroplating method attracted attention, and recently has come to be widely used as an industrial plating method for automobile parts and electronic parts, since the electroplated products have excellent corrosion resistance, aqueous salt solution resistance and solderability.
The plating baths heretofore proposed for the tin-zinc alloy electroplating include, for example, an alkaline cyanide bath, pyrophosphate bath, borofluoride bath, sulfonate bath, carboxylate bath and cyanide-free alkaline bath. Some of them are practically used.
A defect common to the conventional tin-zinc alloy PLATING BATHS is that the current density exerts a strong influence on the composition of the plating alloy. Namely, even when the current density during the plating is fixed, the current density distribution on the surface of the substance to be plated is not always even and, therefore, the composition of the plating alloy is ununiform. This phenomenon is marked particularly when the substance has a large surface to be plated or a complicated shape.
As a result, the properties of the coating and the quality of the plated substance, i.e. the corrosion resistance, chromate coating film-forming properties and solderability, become various.
As an electroplating bath having small influence due to change in current density, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No. Sho 57-2795 proposes a citrate bath containing a water-soluble brightener obtained by reacting phthalic anhydride with a reaction product of an aliphatic amine and an organic acid ester, and the bath is now practically used. J. P. KOKOKU No. Sho 57-2796 also discloses a tin-zinc alloy plating bath containing specified amounts of tin sulfate and zinc sulfate and further citric acid (or its salt), ammonium sulfate and sodium sulfate. Further, J. P. KOKOKU No. Sho 59-48874 discloses a tin-zinc alloy plating bath containing citric acid (or its salt), an ammonium salt and a specified polymer.
However, even when such a bath is used for the electroplating, the uniformity of the alloy composition is insufficient, since the plating alloy composition on certain part of surface of substrate having an extremely low current density has a high tin content. Under these circumstances, special facilities are necessitated and a strict operation control is indispensable at present.